The present invention relates to a novel polycarbonate-made optical article or, more particularly, to an optical article, such as optical disc substrates and optical fibers, made of a polycarbonate resin particularly suitable as a material for forming such optical articles.
Needless to say, acrylic resins are known as a material suitable for forming transparent articles for optical uses such as substrates of optical discs because of their characteristics such as high transparency, high flowability in molding, low birefringence and the like (see, for example, Japanese Patent Kokai No. 56-131654).
However, acrylic resins are generally poor in their heat resistance and shaped articles of the resin are poor in the impact strength and may sometimes be warped upon exposure to moisture.
It has been proposed in Japanese Patent Kokai No. 58-180553 that the above mentioned disadvantages of acrylic resins can be overcome when articles such as optical discs are formed of a polycarbonate resin having a viscosity-average molecular weight of 15,000 to 18,000 as the molding material. Such a high-molecular polycarbonate resin, however, is not quite satisfactory in the flowability in molding and, in particular, is defective in respect of the high birefringence which is very detrimental when formed articles of the resin are used in an optical application. The deficiency in respect of the low flowability in molding can be partly compensated for by using a polycarbonate resin of a relatively low molecular weight or by increasing the molding temperature though being accompanied by various disadvantages such as decreased mechanical strengths of the shaped articles and eventual appearance of stringiness of the molded articles necessarily to cause interruption of the process of continuous molding.
Under such circumstances, various efforts have been made to obtain an optical article made of a polycarbonate resin free from the above described problems and disadvantages in the prior art optical articles made of conventional transparent synthetic resins.